This invention relates to a process of removing sulfur oxides and optionally other gaseous pollutants from flue gases by means of a reactant which is contained in a circulating fluidized bed and contains sodium, potassium, calcium and/or magnesium as a cation and oxide, hydroxide and/or carbonate as an anion, wherein a gas velocity between 1 and 10 meters per second (stated as empty-pipe velocity) is maintained in the fluidized bed reactor, a mean suspension density between 0.1 and 100 kg/m.sup.3 is maintained in the fluidized bed reactor, the mean particle size of the reactant is between 1 and 300 .mu.m. and the quantity of reactant which is circulated per hour is at least 5 times the quantity of reactant contained in the shaft of the fluidized bed reactor.
The flue gases formed by the combustion of fossil fuels may have substantial sulfur contents, particularly sulfur oxide contents, in dependence on the sulfur content of the starting materials. Garbage incinerators installed in continually increasing numbers also produce flue gases, which contain sulfur oxides and owing to the combustion of plastics, which are virtually always present, contain additional contaminants consisting of hydrogen chloride and hydrogen fluoride. Under the regulations for the protection of the environment, such contaminants must be removed from the gases before they are discharged into the atmosphere.
In most of the processes of purifying flue gases the above-mentioned contaminants are removed by a wet scrubbing of the flue gases, particularly by a treatment with solutions or suspensions of substances which react with the pollutants (Ullmann's Encyclopadie der Techn. Chemie, 3rd edition, Volume 2/2 (1968), page 419).
Other processes involve a dry scrubbing process, in which the gases are passed through a stationary pile of solids which react with the contaminants, and consist, e.g., of activated carbon or brown coal coke. Flue gases may also be purified with the aid of a so-called traveling bed of solids, which are progressively laden as they descend in the reactor and are finally discharged while fresh solids are fed to the reactor in its upper portion at the rate at which solids are discharged (Meier zu Kocker "Beurteilung und Aussichten von Verfahren zur Rauchgasentschwefelung", V.G.B. Kraftwerkstechnik 53 (1973), pages 516 et seq.).
Another known apparatus for a removal of pollutants from flue gases by a dry process succeeds the combustion chamber in the direction of flow of the flue gases and is disposed in a boiler portion in which the flue gases have temperatures from 700 to 900.degree. C. That apparatus comprises a fluidized bed, which completely occupies the flow area for the flue gases, and/or a circulating fluidized bed, which is supplied with sorbent consisting of calcium carbonate and/or magnesium carbonate (Published German Application No. 30 09 366). In that apparatus the gas inlet bottom under the fluidized bed is suitably cooled.
It is also known to remove pollutants from flue gases by means of a circulating fluidized bed in a process which is of the kind described first hereinbefore and in which the flue gas is suitably cooled to prevent a rise of the temperature of the mixture of flue gas and reactant above 150.degree. C. (Published German Application No. 32 35 559).
The main disadvantages of wet purification processes reside in that the sludge obtained, which contains sulfites and sulfates and possibly also chlorides, can be disposed of only with great difficulty and that the purified flue gases must be reheated. The known dry purification processes using a stationary or traveling bed have the disadvantage that the sorbents are coarse-grained so that their capacity for combining with the contaminants contained in the flue gas is utilized only to a very low degree and that very large reactors are required because the permissible gas velocity is relatively low and the rates of gas to be purified are high.
Difficulties involved in the removal of pollutants from flue gases at temperatures between 700.degree. and 900.degree. C. by fluidized bed technology reside in that the grate under the fluidized bed must be made of a highly heat-resisting material or provided with a cooling system, which involves a high process expenditure. Besides, the fresh reactant charged in a cold state extracts sensible heat from the flue gas and that heat is no longer available for the generation of steam. Additional measures must be adopted to maintain the flue gases at a temperature between 700.degree. and 900.degree. C. because the flue gases are usually hotter as they leave the combustion chamber and colder as they leave the waste heat boiler. Existing waste heat systems would require to be structurally altered or reconstructed at costs which cannot be justified economically, and special designs are required for that purpose.
Whereas the process disclosed in Published German Application No. 32 35 559 is based on a proper concept, it has the disadvantage that it does not produce satisfactory results under all operating conditions under which flue gases may have to be purified and/or in that it involves a substantial process expenditure.
It is an object of the invention to provide a process which is free of the disadvantages of the known gas purification processes and which can be carried out in a simple manner and with inexpensive reactants and can easily be adapted to all operating conditions of the combustion equipment by which flue gases are delivered.